1. Field of the Invention
The present invention relates to a disk drive and method of manufacturing a disk drive having data sectors written with a compensating offset in the reserved band of the disk surface for improving disk drive performance during initialization.
2. Description of the Prior Art
Hard disk drives store large volumes of data on one or more disks mounted on a spindle assembly. The spindle assembly includes a spindle motor for rotating the disks at a substantially constant angular velocity. Disk drives employ a disk control system for interfacing between a host (e.g., a computer) and the disks to read and write data. Each disk may store data on up to two disk surfaces. On each disk surface, user data is stored in concentric circular tracks (i.e., data tracks) between an outside diameter and an inside diameter of the disk. Servo systems are employed to maintain alignment of a transducer head with a desired target data track (termed "track following"), for reading and writing user data on the disk surface within desired control parameters.
Most current disk drives use embedded servo systems to store servo data on the same disk surface as user data to provide control signals and information employed in the operation of the servo system. User data on each track is divided into data sectors. Embedded servo information is recorded in equally spaced apart servo sectors placed between data regions containing data sectors. For example, a concentric data track may include 60 servo sectors interspersed between data regions. The circular path described by the information in the servo sectors may be termed a servo track.
Each disk surface can be divided into a user data band (or user data area) and a reserved band (or reserved area). The user data band includes the data tracks which are allocated for storing user data on the disk surface. The reserved band is located outside the user data band, and typically comprises a small number of data tracks (e.g., 1 to 20 data tracks) on each disk surface. The reserved band on the disk surface is utilized for storing extensive drive specific data or "optimization" data. The drive specific or optimization data is measured during manufacturing or testing of the disk drive and stored in the reserved band on the disk surface. At power-up, the drive uses a set of default parameters to start operations and reads the optimization data to "tune" the drive for optimum performance. Such parameters may include read channel settings and individual head characteristics including measured read/write head offset.
Presently, disk drives use a dual element transducer head where a magnetoresistive (MR) head is used for reading and a thin film inductive head is used for writing. Inevitably, some offset or skew exists between the two heads due to manufacturing variances such that when the read head is positioned at a given point over the track, the write head is off-track from the read head by some amount. In one known application, when data is written on the disk surface in both the user data band and the reserved data band, the read element is centered on the corresponding servo track. Data is written with an offset from the centerline of the corresponding servo track based on the skew introduced by the dual element head. For subsequent reading of a data track (or reserved data track), the read element is moved off-track from the corresponding servo track center line by the offset amount (termed a "micro-jog") such that the read element is substantially centered over the data track. This results in track following of the corresponding servo track at an offset from the servo track center line during execution of a read command.
In another known application, the micro-jog process can be reversed--that is, the read element can be offset from the servo track centerline to place the write element at the centerline. In this case, track following for a read command will occur at the servo track centerline at each track on the disk. Unfortunately, when track following for the write command at the offset position, the read head may be operating in a less linear region and servo tracking errors may occur which exceed a track mis-registration budget allowed for the servo controller.
The amount of offset varies from transducer head to transducer head and especially between different transducer head manufacturers. As a result of this variance, the offset is measured in manufacturing for each transducer head and saved in the reserved band of the disk surface. One problem is that the disk drive does not know what the offset is at power-up. Accordingly, when a disk drive is powered up, a default offset (in addition to other error recovery steps) is used to read optimization data, including the measured offset information, in the reserved band. The measured offset is subsequently stored in volatile memory for use in reading data stored in the user data band on the disk surface.
Since offsets vary from transducer head to transducer head, it is difficult or virtually impossible to determine a default offset which is suitable for all transducer heads. Further, with decreasing track pitch causing the offset to be more significant, the default offsets may be unsuitable for reading any data from the disk surface. In this case, the disk drive is forced to perform extensive retries and error recovery to find an offset which permits the reserved area to be read for each head and disk surface combination. The number of head and disk surface combinations could be 16 or more in a high capacity drive, therefore the start-up time of a drive may be detrimentally extended. A need exists therefore to improve the performance of a disk drive during start-up when reading optimization data in a reserved area.